Conventional techniques on springs having high fatigue-resisting characteristics have been described, e.g., in JP-A-4-367346 in which the surface of steel wires is electropolished or chemically polished so as to remove micro-defects on it, thereby improving the fatigue resistance or the wires. (The term "JP-A" as used herein means an unexamined published Japanese patent application.) This approach which involves electropolishing or chemically polishing the surface of spring steel wires is effective to a certain extent in improving the fatigue limit since the surface of the wires is made smooth. However, further improvements in the fatigue limit have been impossible on account of the low materials strength.
The present inventors have made an attempt to improve the fatigue resistance of a wire in which some kinds of elements for enhancing the strength of materials are added. However, the attempt at achieving high fatigue resistance by enhancing the materials strength has had the problem in that the increase in materials strength is accompanied by increasing defect sensitivity and that a micro-defect developing on the surface can propagate to cause flexural failure due to fatigue. Hence, it has been impossible to achieve improvement in the fatigue limit beyond a certain value.
The present invention has been accomplished under these circumstances.